The blocker is experimentation and trials. Mathematics and code lives in "virtual" space. Virtual space is nice because everything is infinitely faster than real life.

It really doesn't matter much if AI can make the perfect drug instantly. We will still have to give it to humans, then wait, then observe. Perfect simulation doesn't fix this because the human body is not a mathematical model. It's not just that the human body is really complicated, it's that there are many bodies and how systems interact can't necessarily be predicted ahead of time.

And, even if you could perfectly predict how a drug will function for any arbitrary person, we'd never be okay with just that. No, we'd require cold-hard experiments to show it is safe and effective. There are many, many potential drugs that are, on paper, perfect. But, in reality, worthless.